A TMR sensor serves as a memory element in magnetic devices such as Magnetic Random Access Memory (MRAM) and a magnetic read head. In FIG. 1, a TMR sensor 6 is shown as a stack of layers that has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. In a magnetic read head, the TMR sensor 6 is formed between a bottom shield 5 and a top shield 14. The bottom (seed) layer 7 in the TMR sensor 6 is generally comprised of one or more seed layers that promote a smooth and dense crystal growth in overlying layers. Above the seed layer 7 is an anti-ferromagnetic (AFM) pinning layer 8 and a first ferromagnetic layer that is a “pinned” layer 9 on the AFM layer. The thin tunnel barrier layer 10 above the pinned layer 9 is generally comprised of a dielectric material such as AlOx that may be formed by first depositing an Al layer and then performing an in-situ oxidation. The tunnel barrier layer 10 must be extremely uniform in thickness and oxidation state since small AlOx thickness variations or slight oxidation differences result in large variations in resistance that degrade device performance. A ferromagnetic “free” layer 11 is formed on the tunnel barrier layer 10 and is typically less than 50 Angstroms thick. At the top of the TMR element is a cap layer 12. In a MRAM, the TMR sensor is formed between a bottom conductor and a top conductor
The MTJ stack in FIG. 1 has a so-called bottom spin valve configuration. Alternatively, an MTJ stack may have a top spin valve configuration in which a free layer is formed on a seed layer followed by sequentially forming a tunnel barrier layer, a pinned layer, AFM layer, and a cap layer.
The pinned layer 9 has a magnetic moment that is fixed in the y direction by exchange coupling with the adjacent AFM layer 8 that is also magnetized in the y direction. The free layer 11 has a magnetic moment that is either parallel or anti-parallel to the magnetic moment in the pinned layer. The tunnel barrier layer 10 is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons. The magnetic moment of the free layer may switch in response to external magnetic fields generated by passing a current through the bottom shield 5 and top shield 14. It is the relative orientation of the magnetic moments between the free and pinned layers that determines the tunneling current and therefore the resistance of the tunneling junction. When a sense current 15 is passed from the top shield 14 to the bottom shield 5 (or top conductor to bottom conductor in a MRAM device) in a direction perpendicular to the planes of the TMR layers (CPP designation), a lower resistance is detected when the magnetization directions of the free and pinned layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state. Alternatively, a TMR sensor may be configured as a current in plane (CIP) structure which indicates the direction of the sense current.
A TMR sensor is currently the most promising candidate for replacing a giant magnetoresistive (GMR) sensor in upcoming generations of magnetic recording heads. An advanced TMR sensor may have a cross-sectional area of about 0.1 microns×0.1 microns at the air bearing surface (ABS) plane of the read head. The advantages of a TMR sensor are a higher MR ratio and the preference for CPP geometry for high recording density. A high performance TMR sensor requires a low RA (area×resistance) value, high MR ratio, a soft free layer with low magnetostriction (λ), a strong pinned layer, and low interlayer coupling through the barrier layer. The MR ratio is dR/R where R is the minimum resistance of the TMR sensor and dR is the change in resistance observed by changing the magnetic state of the free layer. A higher dR/R improves the readout speed. For high recording density or high frequency applications, RA must be reduced to about 1 to 3 ohm-um2. As a consequence, MR ratio drops significantly. To maintain a reasonable signal-to-noise (SNR) ratio, a novel magnetic tunneling junction (MTJ) with a MR ratio higher than that provided by a conventional AlOx barrier layer is desirable.
A very high MR ratio has been reported by Yuasa et. al in “Giant room-temperature magnetoresistance in single crystal Fe/MgO/Fe magnetic tunnel junctions”, Nature Materials 3, 868-871 (2004) and is attributed to coherent tunneling. Parkin et al in “Giant tunneling magnetoresistance at room temperature with MgO (100) tunnel barriers”, Nature Materials 3, 862-867 (2004) demonstrated that an MR ratio of about 200% can be achieved with epitaxial Fe(001)/MgO(001)/Fe(001) and polycrystalline FeCo(001)/MgO(001)/(Fe70Co30)80B20 MTJs at room temperature. In addition, Djayaprawira et. al described a high MR ratio of 230% with advantages of better flexibility and uniformity in “230% room temperature magnetoresistance in “CoFeB/MgO/CoFeB magnetic tunnel junctions”, Physics Letters 86, 092502 (2005). However, RA values in the MTJs mentioned above are in the range of 240 to 10000 ohm-um2 which is too high for read head applications. Tsunekawa et. al in “Giant tunneling magnetoresistance effect in low resistance CoFeB/MgO(001)/CoFeB magnetic tunnel junctions for read head applications”, Applied Physics Letters 87, 072503 (2005) found a reduction in RA by inserting a DC-sputtered metallic Mg layer between a bottom CoFeB layer and rf-sputtered MgO. The Mg layer improves the crystal orientation of the MgO(001) layer when the MgO(001) layer is thin. The MR ratio of CoFeB/Mg/MgO/CoFeB MTJs can reach 138% at RA=2.4 ohm-um2. The idea of metallic Mg insertion was initially disclosed by Linn in U.S. Pat. No. 6,841,395 to prevent oxidation of the bottom electrode (CoFe) in a CoFe/MgO(reactive sputtering)/NiFe structure.
Although a high MR ratio and low RA have been demonstrated in MTJs having a MgO barrier layer, there are still many issues to be resolved before such configurations can be implemented in a TMR sensor of a read head. For example, the annealing temperature needs to be lower than 300° C. for read head processing, and rf-sputtered MgO barriers make control of RA mean and uniformity more difficult than with conventional DC-sputtered and subsequently naturally oxidized AlOx barriers. Moreover, a CoFe/NiFe free layer is preferred over CoFeB for low λ and soft magnetic properties but when using a CoFe/NiFe free layer in combination with a MgO barrier, the MR ratio will degrade to very near that of a conventional AlOx MTJ. Thus, a TMR sensor is needed that incorporates a MgO barrier without compromising any desirable properties such as high MR ratio, a low RA value, and low magnetostriction.
A three step barrier layer formation process for a TMR sensor is described in U.S. Pat. No. 6,841,395 and involves sequentially depositing a Mg layer and an oxygen doped Mg film on a ferromagnetic layer and then performing an oxygen treatment. In U.S. Pat. No. 6,828,260, a UV light is used to irradiate a MgO tunnel barrier layer through a transparent overlayer and thereby activate unreacted oxygen in the barrier layer to react with Mg and form a uniformly oxygenated tunnel barrier layer.
A barrier layer comprised of TiOXNY and MgO is disclosed in U.S. Pat. No. 6,756,128. In U.S. Pat. No. 6,737,691, a tunneling barrier layer such as MgO is described with a thickness of <10 Angstroms. No composite barrier layer is disclosed. In U.S. Pat. No. 6,347,049, MgO/Al2O3/MgO and Al2O3/MgO/Al2O3 are disclosed as tunnel barrier layers having low RA values.
A natural oxidation process is used to form a tunnel barrier layer comprised of a single oxide layer in U.S. Pat. No. 6,887,717, U.S. Pat. No. 6,826,022, and in U.S. Pat. No. 6,819,532.